It is well known that high molecular weight thermoplastic resins, for example, polyethylene and ethylene copolymers, may be converted to dispersions of spherically shaped particles which are substantially devoid of particles greater than 25 microns in diameter and in which the number average particle diameter is less than about 10 microns. (Hereinafter, the term "finely divided" will mean such spherical particles.) Thus, McClain U.S. Pat. No. 3,422,049 teaches that such dispersions of finely divided particles may readily be prepared by agitating the molten resin in admixture with water at elevated temperatures and at autogeneous pressure, in the presence of certain dispersing agents which are particular block copolymers of ethylene oxide and propylene oxide. The non-agglomerated spherical particles can be recovered as powders by cooling the dispersions below the fusion point of the resin and collecting the suspended material by filtration or centrifugation.
U.S. Pat. No. 3,418,265 further teaches that the particle size of such thermoplastic resin dispersions can be reduced still further, to the sub-micron level, while retaining the unique spherical particle shape by including in the dispersion process a volatile, inert, water-insoluble organic liquid that is soluble in the thermoplastic resin in an amount between 0.5 and 20 parts per 100 parts of the resin, whereupon a stable, aqueous, film-forming latex is ultimately obtained as the final product. Alternatively, U.S. Pat. No. 3,522,036 teaches that stable, film-forming aqueous latices of high molecular weight polyethylene can also be formed by including a liquid vinyl monomer such as styrene in the dispersion process.
Although the foregoing dispersion procedures are conveniently operated as batch processes, it is also known to produce such finely divided powders in a sequential, continuous dispersion process. See, e.g., U.S. Pat. No. 3,432,483.
U.S. Pat. No. 3,586,654 teaches that it is further possible to conduct the dispersion process in such a way that the polymer particles may be further transformed into spherical particles of controlled average size and size distributions which are the same, larger or smaller than the starting particles. If desired, the dispersion process can be modified in such a manner as to produce spherical foamed particles (U.S. Pat. No. 3,432,801), or to incorporate within the particles certain colorants (U.S. Pat. No. 3,449,291) and pigments (U.S. Pat. No. 3,674,736).
The fine powders are, by virtue of their small particle size, narrow particle size range, and spherical particle shape, unique states of matter which cannot readily be prepared by other conventional processes known in the art. The advantages and utility of such fine powders has been described in many of the aforesaid patent disclosures. In addition, it has been found that various substrates can be coated by applying the above described dispersions of polyolefin fine powders in an inert carrier, heating to evaporate the carrier, and fusing the polyolefin to the substrate (U.S. Pat. No. 3,432,339). Further, U.S. Pat. No. 3,669,922 teaches a process for preparing colored polymer powders having controlled charge and printing characteristics of value as toners in electrostatic printing.
The hydrolysis of ethylene polymers containing hydrolyzable copolymerized units, for example, vinyl acetate units, is also known in the art. Usually, the hydrolysis of such polymers is accomplished by the process of alcoholysis or transesterification in an alcoholic medium in the presence of an acid or basic catalyst, whereby the acetic ester of the alcohol employed is recovered as a by-product. Solution alcoholysis processes, i.e., in which the reaction is carried out in a mixed solvent of an aromatic hydrocarbon and lower primary alcohol, require the use of large volumes of solvent and alcohol and large reactors or kettles. The finished product must be recovered from solution and washed with additional alcohol. The time required to dissolve the resin before hydrolysis is long and the recovery procedure is also time consuming. In addition, the solvent-alcohol mixtures must be separated and recycled for re-use. Solvent losses are incurred which are undesirable from the view point of air pollution as well as for economic reasons. The inherent fire hazard in any process involving organic solvents is a further disadvantage of solution alcoholysis and, additionally, solvent shortages can also present problems.
A different process in which the ethylene-vinyl acetate copolymer in the form of pellets is hydrolyzed by alcoholysis in the swollen solid phase, in a substantially similar alcoholysis media, is described in U.S. Pat. No. 3,510,463. Pressure alcoholysis in the presence of sodium hydroxide in hexane-methanol and methanol alone at temperatures up to 230.degree. C. has also been disclosed. (Japan, 70 33,065, Oct. 4, 1970; Chem. Abstrs. 74, 32375v and Japan. 70 33,066; Chem. Abstrs. 74, 54620y.)
Less commonly, the hydrolysis of ethylene-vinyl acetate copolymers is accomplished by the process of saponification, for example, in an aqueous emulsion where at least a stoichiometric amount of a strong base such as sodium or potassium hydroxide is required for the hydrolysis of a given number of moles of combined vinyl acetate units in the copolymer. See, e.g., U.S. Pat. No. 2,467,774. In general, however, saponification of ethylene-vinyl acetate copolymers is a slow process at ambient temperatures. Thus, Davies and Reynolds, J. Applied Polymer Sci. 12, No. 1, 47 (1968), have reported that at 25.degree. C., 24 hours is required to effect an 18.1% saponification of an ethylene-vinyl acetate copolymer containing 50.8 weight percent vinyl acetate.
Thieme et al., in East German Pat. No. 88,404, describe a one-step dispersion and saponification of ethylene-vinyl acetate copolymers. This process is carried out in an aqueous media at 100.degree.-260.degree. C., under autogeneous pressure, in the presence of a one to two-fold stoichiometrically required amount of sodium or potassium hydroxide, optionally in the presence of an inert, water-insoluble softening agent such as xylene, and in the presence of a surface active agent such as an alkyl sulfonate, an alkyl aryl sulfonate, an acyl derivative of N-methyl taurine, or an alkali salt of a higher carboxylic acid, or a non-ionic surfactant based on ethylene oxide (ethoxylated fatty alcohol). The East German patent does not disclose the particle size range of the product when the process was carried out without any added softening agent or surfactant. In fact, the product was actually milled before classification, and even after milling, the particle size range of the milled product was predominantly (97%) in the range of 125-1252 microns. When a surfactant was used, the unmilled classified product was still comparatively coarse. In the most favorable instance disclosed, employing an N-alkyl sulfonate surface active agent, 78.3% of the particles had diameters in the range of 125-1000 microns, whereas only 21.7% of the particles had diameters below 125 microns.
It is evident from the East German patent that the so-called "finely granulated" product produced therein is much coarser than the finely divided powders produced, for example, in the aforesaid McClain U.S. Pat. No. 3,422,049.
We have found that when the dispersion and saponification process of the East German patent is carried out using the ethylene oxide-propylene oxide block copolymers of the McClain patent, finely divided dispersions of the hydrolyzed copolymer are not obtained. Indeed, the particles have dimensions of 100 microns or more, and a portion of the particles are found to be irregular in shape (short fibers, agglomerates). Surprisingly, we have discovered that such finely divided dispersions can be obtained only if the system contains residual salts in amounts no greater than about 0.1 weight percent calculated as the cation. This discovery was particularly surprising in view of the fact that the Thieme et al reaction requires stoichiometric or greater amounts of the strong inorganic base for the amount of combined vinyl acetate to be saponified on a molar basis. Accordingly, if Thieme et al's process is operated in the presence of the disclosed concentrations of water, polymer and surfactant and with sufficient inorganic base to produce significant amounts of saponification, the product will necessarily be a coarse granular material while if the amount of base employed is reduced to that corresponding to about 0.1% (as cation) or less, there would not be enough base present to produce any significant amount of saponification.
Accordingly, it is the object of this invention to provide a process for the saponification and dispersion of ethylenevinyl acetate interpolymers into substantially spherical particles having an average particle size of about 10 microns or less and substantially devoid of particles greater than about 25 microns in size. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.